1. Field of the Invention
The present invention relates to a magneto-optical recording medium having a high areal recording density. In particular, it relates to a magneto-optical recording medium having a high sensitivity to a recording magnetic field.
2. Description of the Related Art
A variety of magnetic recording media have been in practical use as rewritable recording media. In particular, magneto-optical recording media have been deemed promising as mass-storage media capable of high-density recording. There is a known magneto-optical recording medium in which information is recorded by writing magnetic domains in a magnetic thin film by thermal energy of a semiconductor laser and information is read out by the magneto-optical effect. In recent years, along with the trend of digitization of moving images, there have been demands for a larger-capacity recording medium having increased recording density.
Generally, the linear recording density of an optical recording medium strongly depends on the laser wavelength used in a readout optical system and on the numerical aperture (NA) of an objective lens. More specifically, once the laser wavelength λ used in the readout optical system and the numerical aperture (NA) of the objective lens are determined, the diameter of the laser beam is also determined. Thus, the detection limit of the spatial frequency capable of reading signals from recorded pits is about 2NA/λ. Therefore, to achieve high-density recording in optical disks, in general, it is necessary to decrease the laser wavelength used in the readout optical system and to increase the numerical aperture NA of the objective lens. However, it is difficult to decrease the laser wavelength due to heat generation and inefficiency of the light-emitting element. Moreover, increasing the numerical aperture of the objective lens results in a shallow depth of focus, thus requiring higher mechanical accuracy. Hence, super-resolution techniques have been developed in various forms to improve the recording density by devising a structure and a readout method that do not change the laser wavelength and the numerical aperture of the objective lens.
For example, Japanese Patent Laid-Open Nos. 3-93058 and 6-124500 disclose a method for reading a signal. A signal is recorded in a memory layer, which is magnetically coupled to a readout layer, the memory layer adjoining or contacting the readout layer. Simultaneously, the magnetization of the readout layer is oriented. In the case of Japanese Patent Laid-Open No. 6-124500, the magnetization is oriented in-plane. Then, a laser beam is directed on the readout layer, causing heat generation. A recorded signal in the memory layer is transferred to the heated area of the readout layer and the transferred signal is read out. In this method, the size of the area (aperture) heated by the laser beam to reach a transfer temperature (the area having a signal to be detected) can be smaller than the spot diameter of a readout laser beam. Therefore, intersymbol interference is reduced during readout and a signal having a pit period with an optical detection limit of λ/2NA can be read out. Such a readout method is referred to as a magnetically-induced super resolution readout method (MSR).
In MSR, however, the signal detection area is small relative to the spot diameter of the readout laser beam, thus causing a significant decrease in the amplitude of the readout signal. Accordingly, the readout output is generally insufficient. Therefore, the signal detection area cannot be much smaller than the spot diameter. As a result, it is not possible to accomplish a significantly high recording density relative to the recording density determined by the diffraction limit of the optical system.
Japanese Patent Laid-Open No. 6-290496 discloses a magneto-optical recording medium and a readout method. A magnetic domain wall (hereinafter, referred to as “domain wall”) at the boundary of a recorded mark shifts to higher temperatures by applying a temperature gradient. A signal recorded at a high density exceeding the readout resolution limit of the optical system can be read without a decrease in the amplitude of the readout signal by detecting the displacement of the domain wall. Such a readout method is referred to as a domain wall displacement detection (DWDD) readout method.
As shown in FIG. 7A, in the DWDD readout method, a first magnetic layer 701 having a small domain-wall coercive force, a second magnetic layer 702 having a low Curie temperature, and a third magnetic layer 703 having a large domain-wall coercive force, are stacked. As described in J. Magn. Soc. Jpn., 22, suppl. No. S2, pp. 47–50 (1998), the first magnetic layer 701 functions as a displacement layer (readout layer) where a domain wall moves during readout, the second magnetic layer 702 functions as a switching layer for controlling a starting position of the domain wall displacement, and the third magnetic layer 703 functions as a memory layer for retaining information. Forming a temperature distribution on the second magnetic layer 702 by laser beam irradiation (see FIG. 7B) leads to a distribution of the domain wall energy density (see FIG. 7C). Since higher-temperature regions have low domain wall energy densities, a driving force for moving a domain-wall to the higher-temperature region is generated.
However, these magnetic layers are magnetically coupled to each other by an exchange coupling force at temperatures below the Curie temperature of the switching layer; hence, a domain wall cannot move by the domain-wall coercive force of the memory layer, even when the driving force is applied. Since the exchange coupling force decreases at a position having a temperature (Ts) near the Curie temperature of the switching layer, only a domain wall in the displacement layer, having a small domain-wall coercive force, can move to a higher-temperature region. When the recording medium moves in a region having a temperature distribution, such a domain wall movement occurs at time intervals corresponding to the spacing between adjacent domain walls. Therefore, a signal recorded at a high density exceeding the readout resolution limit of the optical system can be read by detecting the domain wall movement.
A problem with the medium used for the DWDD readout method described above is that decreasing the length of a recorded mark impairs sensitivity to a recording magnetic field. This is because magnetic layers having as low a saturation magnetization as possible are used in order to reduce the effect of a stray field on the domain wall displacement during readout. In addition, the memory layer is composed of a magnetic material having a high magnetic anisotropy in order to reliably store a shorter recorded mark; hence, large energy is required for reversing the direction of magnetization.
In magnetic field modulation recording, which is suitable for higher-density recording, a magnetic head can generate a magnetic field ranging in intensity from about 200 to about 300 Oe at best. In view of lower power consumption and higher-speed, the magnetic field generated by the magnetic head preferably has an intensity of 200 Oe or less.
To solve the problem described above, the following technique has been widely employed: a magnetic layer having weak magnetic anisotropy relative to that of the memory layer is provided adjacent to the memory layer, thus resulting in a high sensitivity to a recording magnetic field.
The magnetic layer having a low magnetic anisotropy (hereinafter, referred to as “auxiliary memory layer”) is provided to aid in recording at temperatures required for recording, i.e., high temperatures. An increase in the saturation magnetization of the auxiliary memory layer inevitably increases the saturation magnetization of the same layer at low temperatures. As a result, the intensities of a demagnetizing field and a stray field also increase, thereby affecting recording. That is, a variation in the length of a recorded mark caused by the effect of the demagnetizing field and the stray field leads to a pattern-dependent shift of a domain wall, thus impairing the jitter properties in reading a random signal. In land/groove recording, this problem becomes more serious.
In a medium used for the DWDD readout method, adjacent domain walls need to be isolated from each other in order that the domain wall in the displacement layer can readily move during reading. To achieve the isolation of the adjacent domain walls, after forming the memory layer, the switching layer, and displacement layer, these layers, which are disposed on a guide groove between recording tracks, are subjected to annealing treatment with a high-power laser beam. That is, the exchange coupling force between the magnetic layers on the guide groove decreases or is removed by the annealing treatment. The domain wall isolation is accomplished by extending the recording mark to the guide groove.
However, when the land/groove recording and narrower track pitch are applied to achieve higher-density recording, the annealing treatment for a guide groove between recording tracks is quite difficult, thus causing problems in reliability.
For the land/groove recording, Japanese Patent Laid-Open No. 9-161321 discloses the following: an increase in groove depth can delimit the magnetic layers up to a certain point, thereby eliminating the need for the annealing treatment.
However, as shown in FIG. 8, in this method disclosed in Japanese Patent Laid-Open No. 9-161321, a film 801 deposited on a groove 802 has a thickness lower than that of a film deposited on a land 803; hence, another problem arises in that the two films deposited on the groove and the land have different properties, especially, in sensitivities to the recording magnetic field.
Consequently, in such a medium for the DWDD readout method, there are conflicting requirements in terms of sensitivity to the recording magnetic field of a minute mark and the jitter properties of a random signal; therefore, it is difficult to balance them.